Divergent iron dissolution pathways controlled by sulfuric and nitric acids from the ground-level to the upper mixing layer

Abstract. Iron (Fe) plays a crucial role in the global biogeochemical cycle, marine ecosystems, and human health. Despite extensive research on Fe dissolution, the understanding of the mechanism of the Fe acidification process remains highly controversial. Here, we revealed significant differences in Fe acid dissolution between the upper mixing layer and the ground-level of a megacity. The results showed that air masses with elevated n[SO42-] / n[NO3-] ratios (5.4 ± 3.7) yielded more enhanced iron solubility (%FeS, 8.7 ± 2.4 %) in the upper mixing layer after atmospheric aging compared to those (1.6 ± 0.7 and 3.3 ± 0.4 %, respectively) at the ground-level near source regions of acidic gases. Further analysis suggested that Fe dissolution is primarily driven by sulfuric acid in the upper mixing layer different from nitric acid at the ground-level, attributing to the aging processes of acidic species during long-range transport. %FeS also exhibits a clear size dependence: sulfuric-acid dominates in submicron aerosols (Dp < 1 µm), leading to elevated %FeS (3.5 ± 3.9 %), whereas alkaline mineral dust in supermicron particles (Dp > 1 µm) neutralizes nitric acid and suppresses Fe dissolution (1.8 ± 2.2 %). This finding highlighted that sulfuric acid dominates Fe acidification process in the upper layer and submicron particles, but the contribution of nitric acid to Fe dissolution at the ground-level is equally important. Our study provides new data sets for testing atmospheric model's capability to simulate dissolved Fe concentration and deposition and will help to improve the accuracy of Fe solubility predictions.